1. Field of the Invention
This invention relates to the preparation of high temperature resistant, insulating polyisocyanurate foams and is more particularly concerned with a novel catalyst combination for the preparation of such foams from compositions containing polyester polyols and blowing agents used to replace the conventional fully halogenated chlorofluorocarbons.
2. Description of the Prior Art
It is well known in the manufacture of rigid polyurethane and polyisocyanurate foams to employ chlorofluorocarbons, such as trichlorofluoromethane, as the blowing agent. These chlorofluorocarbon compounds boil or exhibit a significant vapor pressure at ambient temperatures and are volatilized during the exothermic reaction of an isocyanate with an active hydrogencontaining compound, such as a polyol. The expanding gas is entrapped within the reaction mixture and forms an insulating cellular structure. While the foam industry has had good results using the conventional chlorofluorocarbon blowing agents, such as CFC-11, the agents have come under attack in recent years on the ground that they are believed to give rise to environmental problems concerned with ozone depletion in the stratosphere. Accordingly, the search is ongoing for alternative blowing agents with a low ozone depletion factor to replace the conventional ones.
It is believed that hydrogenated CFC's (also known as HCFC's), which are partially halo-substituted hydrocarbons, present less risk than the CFC's. Because the HCFC's contain one or more hydrogen atoms, they more readily dissociate under conditions encountered in the atmosphere, and therefore, less of them would reach the ozone layer of the stratosphere in a form which could cause significant damage. Accordingly, the hydrogen-containing halocarbons have been investigated as possible alternatives for CFC-11 in rigid foam applications.
The search for acceptable alternative blowing agents among hydrogenated CFC's is difficult because of the combination of performance characteristics required of them. The agents must not impart unacceptable fire or toxicological risks in the foaming operation or to the finished foam products. In this regard, it is desirable for polyisocyanurate foams to develop a thick protective char under fire exposure, but it is found that the char shrinkage for foams made with HCFC's is greater than for identical polymer compositions blown with CFC-11. The agents further must not react with the other components of the foam formulation and should be adequately soluble in the foam system. Also, their boiling point, vapor thermal conductivity, capacity to efficiently produce gas and diffusion rate must be appropriate for the formation of highly insulating foams. Finally, the alternative blowing agents should be reasonable in cost.
There still remains a need for a rigid polyisocyanurate foam which has superior properties even though the conventional CFC blowing agents are avoided in its production.